1. Field of the Invention
This invention relates to a device for supporting an inner or primary pipe within an outer containment pipe of a double containment pipe assembly and, more particularly, a support for a pipe which can be constructed of a different material than the outer or containment pipe.
2. Background Information
Hazardous fluids cannot be routinely conveyed in open top drainage channels, but instead in enclosed pipes or conduits. Recognizing that such pipes and conduits can leak because of manufacturing defects, excessive pressure, corrosion or other reasons, the prior art has devised dual containment systems which generally involve the placement of a relatively small fluid carrying closed-wall conduit or pipe within a larger closed-wall conduit or pipe which does not carry fluids except in the event of a leak of liquid carried by the inner conduit. The smaller conduit or pipe is supported by resting directly on the lower, inner wall of the outer conduit or pipe. Alternatively, supports have been proposed wherein the inner pipe may be supported in spaced relation to the lower inner wall of the outer conduit or pipe. Examples of such supports are illustrated in U.S. Pat. Nos. 4,751,945; 3,863,679; and 3,417,785.
While such supports may be used to space the inner and outer pipes and provide ready access to the inner pipe as spaced locations along the outer pipe, the supports shown in the prior art do not lend themselves to support pipes constructed from two entirely different materials. With such a support, the inner and outer pipes may be rigidly connected together by welding them to the same support.
Often, it is desirable to use weldable, dissimilar, thermosplastic materials for the inner and outer pipes. These include crystalline and partially crystalline thermosplastics from the vinyl, polyolefin, fluoropolymer and polyketone families. When a combination of these materials are to be used, the inner and outer pipes may be welded together at the end of each segment of pipes or fittings, directly at each of the unique supports of the present invention. Once the ends of pipes and/or fitting segments are rigidly attached together by welding them to the supports, the dissimilar inner and outer pipes may subsequently be joined in a simultaneous fashion with the use of butt welding techniques.
An example of where this arrangement would be beneficial is where polypropylene, a commonly used material for acid/caustic piping, is used as the material of construction of the primary or inner piping. Normally, a polypropylene material would be used as the material of the secondary or outer, containment piping as well, if a simultaneous fusion method is to be the desired method of joining. However, the use of high density polyethylene piping for the outside piping would represent a substantially less expensive selection. Therefore, a combination of polypropylene/high density polyethylene pipes that would be capable of being simultaneously welded would represent a highly economical selection. Since the temperatures of welding and the specific welding pressures of the materials are very similar, this is entirely feasible and very readily capable of being done.
Other features of this support that make it desirable have to do with the structural and mechanical aspects of its design. The support is designed so that it supports the inner pipe in such a way that loads between the inner and outer piping are very evenly distributed The magnitude of these loads are kept to a minimum, and in any case there is the complete avoidance of point loads created on either the primary and secondary, or outer, containment piping.